Consider the reaction `A+ B ↔ E` . The equation illustrates what happens in the total reaction but does not demonstrate what is going on in the intermediate steps. One possible set of reactions can be this:
`(1): A + B ↔ C + D`
`(2): C+D ↔ E`
The net reaction is still `A+B ↔E` , but the reaction is shown to take place in two separate reactions. Reactants `C` and `D` are called intermediates as they are formed and consumed during the reaction. Each of the reactions `(1)` and `(2)` are called elementary steps.
Let's go back to the example above. One of these elementary steps must be slower than the other. This step is called the rate-determining step and determines the rate of the entire reaction.
Think of it this way: in a single lane, the speed at which cars can drive is limited by the slowest car. If the first car is driving at `20 "mph"` , all cars behind the car must also drive at or below `20 "mph"` . If the first car drives at `80 "mph"` , the rest of the cars behind can now drive at or below `80 "mph"` . The rate-determining step is the same concept; the reaction can only proceed as quickly as its slowest elementary step.
Going back to the original reaction `A + B ↔ E`
If we knew that `(1)` was the slower reaction, we could write the elementary steps as:
`A + B ↔ C + D ;("slow")`
`C + D ↔ E ;("fast")`
Since the rate of the reaction depends on the slowest step, the rate law would be the same as the rate of the slower step. The overall rate of the reaction would therefore be written as
`"rate"= k[A][B]`
This seems pretty intuitive for this reaction, but for many reactions involving elementary steps, the rate law is not what one would imagine. For example, the reaction of nitrogen dioxide and carbon monoxide:
`NO_2 + CO ↔ NO + CO_2`
The reaction mechanism is predicted to be:
`(1) NO_2 + NO_2 ↔ NO_3 + NO ;("slow")`
`(2) NO_3 + CO ↔ NO_2 + CO_2 ;("fast")``
Since the rate law is determined by the rate determinine step `(1)` , the rate law is written as:
`"rate"=k[NO_2]^2`
Which one would've never gotten from the overall equation.
According to the current limitations of science, a reaction mechanism can never be proven to be true (see Fun Facts section for commentary). Instead, we can only say that a reaction mechanism is "likely correct," and there are two conditions that a mechanism must adhere to be if it is to be likely correct.
1. The elementary steps of the mechanism must add up to the balanced equation.
2. The reaction mechanism must agree with the experimentally determined rate law.
If we had reaction `A + B ↔ C` with the elementary steps
`(1) A + B ↔ E`
`(2) E↔ F+ G`
The elementary steps `(1)` and `(2)` clearly don't add up to the balanced equation. Thus, the mechanism consisting of `(1)` and `(2)` cannot be said to be likely correct.
Likely, if we had a reaction `A + B ↔ C` with the predicted mechanism
`(1) A+B ↔ E ("fast")`
`(2) E↔C ("slow")`
and we experimentally determined that the rate law is
`"rate"=k[A]^2`
we know that the predicted mechanism cannot be true as the mechanism predicts `"rate"=k[E]` .
Only when a mechanism fulfills both conditions can the mechanism be said to be likely correct. There can be multiple mechanisms that satisfy the two conditions, so it takes a lot of experiments to determine which mechanism is the "likeliest" to be correct.
The way that scientists predict mechanisms is by creating a list of possible mechanisms, ruling out a large number according to fundamental principles or observations, and finally experimentally performing the reaction and recording the rate. By comparing the rate to the remaining list of possible mechanisms and keeping only the ones that adhere to the two principles, they're able to theoretically come up with a mechanism for a reaction. These mechanisms can be further tested to determine their validity.
1. A reaction mechanism is a predicted series of elementary steps.
2. Reaction mechanisms can never be proven correct.
3. The rate law of a reaction is going to be equal to the rate of the rate-determining step.
4. In order for a mechanism to be "likely correct," it has to add up to the balanced equation and agree with the experimentally determined rate law.
#1. Why can't reaction mechanisms be determined to be correct?
Paper "Can Reaction Mechanisms Be Proven?" by Allen Buskirk and Hediyeh Baradaran